The principle of the cyclic extrusion compression (CEC) process developed by Korbel et al. is represented schematically in Fig. 21 [18,108–111]. In the CEC process, a sample is contained within a chamber and then extruded repeatedly backwards and forwards. This process was invented to allow arbitrarily large strain deformation of a sample with preservation of the original sample shape after n passes. The accumulated equivalent strain is approximately given by ε=4nln(D/d) (6) where D is the chamber diameter, d the channel diameter and n the number of deformation cycles. Since the billet in the CEC process is compressed from the both ends, a high hydrostatic pressure is imposed. The extrusion–compression load becomes high so that the special pre-stressed tools are required, otherwise the tool life will be short. This process is better suited for processing soft material such as aluminum alloys. However, the strain introduced in the forward extrusion may be cancelled by the strain introduced on the backward extrusion.
Transcript
The principle of the cyclic extrusion compression (CEC) process
developed by Korbel et al. is represented schematically in Fig. 21
[18,108–111]. In the CEC process, a sample is contained within a
chamber and then extruded repeatedly backwards and forwards.
This process was invented to allow arbitrarily large strain
deformation of a sample with preservation of the original sample
shape after n passes. The accumulated equivalent strain is
approximately given by
ε=4nln(D/d)
(6)
where D is the chamber diameter, d the channel diameter and n the
number of deformation cycles. Since the billet in the CEC process is
compressed from the both ends, a high hydrostatic pressure is
imposed. The extrusion–compression load becomes high so that
the special pre-stressed tools are required, otherwise the tool life
will be short. This process is better suited for processing soft
material such as aluminum alloys. However, the strain introduced
in the forward extrusion may be cancelled by the strain introduced
on the backward extrusion.
CIRP Annals
Cyclic Extrusion andCompressionCyclic extrusion and compression(also sometimes called “hourglasspressing”) is performed by pushing asample from one cylindrical chamber ofdiameter do to another with equal dimensionsthrough a die with diameter dm
which is markedly smaller than do;23 theprinciple is illustrated in Figure 5. Thus,the processing induces extrusion and thechambers provide compression so that,during one cycle, the material is pushedto fi rst experience compression, thenextrusion, and fi nally compression again.The true strain produced in one cycle iscalculated as Δε = 4 ln (do/dm). In thesecond cycle, the extrusion direction isreversed, leading to the same sequence ofdeformation modes. The process can berepeated N times by pushing the sampleback and forth to give an accumulatedtrue strain of (NΔε). With a diameter
ratio of typically dm/do ≈ 0.9, the strainimposed on the material in one cycle isΔε ≈ 0.4. Accumulated true strains of upto 90 have been reported23 with sampledimensions of about 25 mm in lengthand 10 mm in diameter. The deformationspeed is as low as ~0.2 mm/s in orderto limit heating of the specimen to <5K. Although the strains reached withthis method are much higher than thosewith any unidirectional SPD technique,the microstructure and/or mechanicalproperties are similar because of theextra annihilation of dislocations due tothe cyclic character of the straining.24
Producing Bulk Ultrafi ne-GrainedMaterials by Severe PlasticDeformationRuslan Z. Valiev, Yuri Estrin, Zenji Horita, Terence G. Langdon, Michael J. Zehetbauer,
and Yuntian T. Zhu
Schematic distribution of radial and axial stressesin reciprocical extrusion process
PROCESSING OF METALS BY SEVERE PLASTIC DEFORMATION
(SPD) – STRUCTURE AND MECHANICAL PROPERTIES RESPOND
